Infra-red emitting decoy flare

ABSTRACT

An infra-red emitting decoy flare capable of diverting an incoming missile equipped with a counter-countermeasures system away from an intended target consisting of a primer flare ( 2 ), a spectral flare ( 4 ) and a means for igniting the primer flare ( 22, 30 ), all contained within a flare casing ( 6 ). The primer flare ( 2 ) is formed from a fast burning pyrotechnic composition and is adapted to produce an intense infra-red source of short duration on ignition. The spectral flare ( 4 ) is ignited by the burning of the primer flare ( 2 ) and is adapted to produce a slower burning composition having a fixed ratio in the intensity of infra-red radiation emitted, when burning, in at least two fixed bands.

This invention relates to an infra-red (IR) emitting decoy flare capableof being launched from a target to divert a missile equipped with an IRseeker system away from that target, and particularly to an IR emittingdecoy flare capable of diverting a missile having a seeker operatingwith a counter-countermeasures (CCM) system using a spectraldiscriminator.

Most infra-red seeker systems operate in a certain wavelength range, orband, of the infra-red spectrum. In this band, the radiated energy fromnon natural sources is generally easy to detect and the hot componentsof aircraft exhausts or tank engines, for example, radiate strongly,enabling targets to be easily identified and located.

Known decoy flares conventionally comprise pyrotechnic compositionsbound together with an organic binder and pressed to form pellets. Whenan incoming missile is detected a pellet is launched from the target andignited. The pellet burns over its surface to produce an intenseinfra-red source in this band, which can lure the infra-red seekersystem of the missile away from the target.

UK patent application GB 2,300,035 describes an infra-red decoy flarewhich is formed from a pyrotechnic composition which burns to emitinfra-red radiation. The composition is formed into a plurality ofdifferent blocks with different volumes and different surface areas soas to have different rates of burning. Ignition of all the blocksproduces an infra-red source which is intense enough to cause themissile to lock onto the flare. After a short time the aircraft will beoutside of the field of view of the missile and some of the fast burningblocks will burn completely away. The flare will then radiatecomparatively weak radiation for a time in order to complete thediversion of the missile.

However, advances in missile seeker systems and CCM systems have led toseeker systems being able to recognise a decoy flare and ignore it. Someadvanced seeker systems are equipped with CCM systems that compare theratio of the intensity of IR radiation in one band with the intensity ofIR radiation in another band of the IR spectrum. Due to the temperaturedifference between a conventional flare and the radiating parts of atypical target and the corresponding different ‘grey body’ radiationspectrums, the CCM system can identify and disregard the flare.

It is therefore an object of the invention to provide a decoy flarewhich alleviates at least some of the aforementioned disadvantages andwhich is capable of diverting a missile equipped with an infra-redseeker system and a spectral counter-countermeasures system away fromits intended target.

Thus according to the present invention there is provided an infra-redemitting decoy flare comprising a flare casing, two pyrotechniccomponents housed within the flare casing and an ignition means forigniting the pyrotechnic components characterised in that the twopyrotechnic components comprise a primer flare and a spectral flarewherein the primer flare consists of at least one primer pellet, eachprimer pellet being composed of a fast burning pyrotechnic compositionand being adapted such that, in use, ignition of one primer pelletcauses rapid ignition of all the primer pellets to produce an intenseinfra-red source, wherein the spectral flare consists of at least onespectral pellet, each spectral pellet being of a pyrotechnic compositionadapted such that, in use, ignition of the spectral pellets produces aspectral infra-red source wherein the ratio of the intensity of theinfra-red spectrum at at least two fixed bands is within a fixed range,and wherein the primer flare and spectral flare are adapted such that,in use, the spectral flare is still burning after the primer flare hasfinished burning.

In use, the ignition bf the primer flare creates an intense IR source ofshort duration. The sudden increase in energy can trigger a missile'sCCM system, Due to the short duration of burning of the primer flarehowever, by the time the missile's CCM system is active the primer flarewill have stopped burning but the spectral flare will still be burning.The spectral flare has pre-set ratios of intensity between differentbands of the IR spectrum and therefore appears to the missile's CCMsystem to have the intensity ratios that an intended target would have.Indeed, due to the varying aspects that a target may present to amissile seeker and the fact that the ratio of intensities of thedifferent bands alters when viewing a target from a different angle, sayan aircraft head on as oppose from the rear, the spectral flare may bejudged by the missile seeker and CCM system to be more target like, interms of the required ratio of intensities at different bands, than theactual target itself.

Preferably the spectral flare is adapted such that at the fixed bands ofthe IR spectrum the flare is more intense than the intended target. Thespectral flare will then be the most intense IR source with the correctspectral characteristics. Further, the very intense radiation from theprimer flare can saturate some missiles seeker systems. This would notonly cause a missile to activate its spectral CCM system but could alsocause automatic brightness compensators to come into operation. Afterthe primer flare has stopped burning the automatic compensators willstart to reduce to their previous levels. However, if the intensity ofthe spectral flare in the bands measured by the missile is greater thanthat of the intended target then the intensity compensators of themissile seeker may not reduce to a level that would include the target.Therefore the spectral flare will be the only object with the correctspectral characteristics in the field of view of the missile.

Also, on initial ejection the primer pellet lights up and burnsextremely quickly. Thus the flare will still be close to the target onignition and energy from the burning flare will be reflected from thesurface of the target. This can increase the radiation seen by themissile's seeker system. Further, the radiation reflected from thetarget can cause the target to appear to be flare like to the seekersystem thus prompting the missile to actually ignore the target.

In order to achieve a fast burn rate the primer pellets are preferablydiscs and the primer flare consists of a stack of said discs. Bydividing the primer flare into discs the surface area available forburning is increased over that of a single pellet of the same dimensionsas the stack. The bum rate is therefore correspondingly increased. Thediscs are also preferably provided with a central hole which againincreases the bum rate, but also aids in rapid ignition of the primerflare by allowing the passage of hot particles through the stack.Alternatively a single primer pellet is used and is provided withplurality of holes through the pellet. This again increases the surfacearea for burning and increases the burn rate. Another means ofincreasing the burn rate is providing the primer pellet or pellets withdeep grooves to create more burning surface area. Other arrangements forthe primer pellet will be readily apparent to the skilled addressee.

A fast burn time is required so that the primer flare has finishedburning by the time that the seeker system has adjusted. The burn timeof the primer flare is therefore preferably between 100-600 ms, morepreferably between 150-250 ms. Too short a burn time however can reducethe efficiency of the primer flare as there would be insufficient timefor efficient combustion processes to occur.

The primer flare is conveniently comprised of a composition of anoxidisable metallic material, an oxidising halogenated polymericmaterial and an organic binder. Suitable metallic fuels are well knownin the art and include magnesium, aluminium, alloys of magnesium oraluminium, titanium, boron and zirconium. Preferable the oxidisablemetallic material is magnesium. When ignited magnesium undergoes anenergetic and vigorous exothermic reaction with halogenated polymers andtherefore is particularly suitable for the heat and speed of combustionrequired.

Similarly the oxidising halogenated polymeric material used in preferredcompositions for the primer flare is a fluorinated polymer becausefluorine is a better oxidising agent than other halogens and thereforewill react more vigorously and create a more intense IR source. Suitablefluorinated polymers include polytetrafluoroethylene (Teflon (TM) orPTFE) and its copolymers with perfluoropropylene,polytrifluorochloroethylene, copolymers of trifluoroethylene withvinylidene fluoride, homopolymers of perfluoropropylene and copolymersof perfluoropropylene with vinylidene fluoride, homopolymers ofhexafluoropropylene and copolymers of hexafluoropropylene withvinylidene fluoride. PTFE is particularly suitable as it has a highpercentage of fluorine in it.

Suitable organic binders are well known in the art and includepolyvinylchloride, straight chain chlorinated paraffins such asAlloprene (TM) or Cereclors (TM) and the tripolymer of vinylidenefluoride, hexafluoropropylene and tetrafluoroethylene. Fluorinatedorganic binders are advantageous in that the binder, also being anoxidising agent, will join in the reaction. A preferred binder is acopolymer of vinylidene fluoride and hexafluoroethylene, for exampleVRFON A (TM), which coats and binds the constituents very well as wellas adding to the reaction.

A preferred composition for the primer flare is therefore amagnesium-Teflon-Viton (MTV) composition. The ratio of the constituentswill be chosen so that there is the smallest amount of unreactedmaterial after combustion, allowing for the amount of atmospheric oxygenpresent that will join in the reaction in a particular flareapplication. The ratio of the constituents will be easily determined bythe skilled person.

As the spectral flare needs to have a longer burn time than the primerflare the spectral flare may usefully be formed from a single pellet.The spectral pellet can therefore bum relatively slowly andconsistently.

In order to ensure that the intensity of the spectral flare is brightenough the spectral pellet may be provided with a central hole throughthe pellet. The hole will not only increase the intensity of theradiation from the spectral pellet by providing an internal burningsurface but with also ensure consistency of the radiation. As theintensity of the flare is related to the surface area of the burningpellet, a pellet burning from the outside only will slowly drop inintensity as the surface area of burning decreases. Having a centralhole, however, means that as the outside surface area of burningdecreases, the internal burning surface increases, resulting in arelatively consistent burn, increasing the viability of a flare to bemistaken as the target by the missile system. A central hole will alsoaid in rapid and consistent ignition of the whole of the spectral flare.

Conveniently the spectral flare and primer flare may be adapted suchthat, in use, the spectral flare is ignited by the burning of the primerflare.

The spectral pellet may advantageously be formed to produce quantitiesof hot gas. Conveniently the spectral pellet may be formed from anorganic fuel, an oxidant and a binder. Organic fuels decompose toproduce gases such as carbon dioxide which can be similar to theelements produced by an aircraft engine say. Thus use of an organic fuelcan provide the required spectral characteristics. Suitable fuelsinclude organic compounds such as sucrose, lactose or starch and alsocompounds such as potassium benzoate. As an alternative to organic fuelsthe spectral pellet may be formed from a boron fuel with a suitableoxidant and binder.

Suitable oxidants include potassium perchlorate, potassium nitrate,sodium nitrate or ammonium nitrate. Suitable binders include Viton A,dextrin or polybutyl rubber although organic binders are preferred asagain they decompose into relevant gasses thus binders such as Viton Aor GAP are preferred. Particularly advantageously however explosivematerials with a waxy composition may be used as binders. Suchexplosives will be able to function as binders due to their consistencyand will add to the energetic reaction on ignition. Suitable explosivesinclude RDX, HMX and HNS and can also add to the hot gasses produced bythe fuel and oxidant. Other oxidants and binders may be used however andcould be easily determined by the skilled person.

One advantageous spectral composition comprise approximately 30% byweight potassium benzoate, 65% by weight potassium perchlorate and 5% byweight of binder, say Viton A or RDX.

Another advantageous spectral composition has, excluding binder,approximately 30% by mass of boron and 70% by mass of potassium nitrate,with Viton A as a binder in a sufficient amount as could be easilydetermined by one skilled in the art. The composition may also includeother materials to enhance the spectral effect. Another advantageouscomposition has, again excluding binder, 20% by mass of boron fuel with70% potassium nitrate and 10% by mass of silicon.

In some instances it will be beneficial that the primer flare burn withsome visible component or without a spectral characteristic, say whenutilising reflection from the target surface. However, in othercircumstances it would be advantageous that the primer flare also bumwith a spectral component. In such instances the primer flare maycomprise a fast burning spectral composition. The actual compositioncould be the same as could be used or the spectral pellet but to ensurea fast light up and generation of sufficient energy grooves or aplurality of discs would be used and smaller particle sizes would beused as is well understood in the art. Explosive binders wouldpreferably be used to add to the intensity produced by the primerpellet.

Further advantages and embodiments of the invention will now bedescribed by reference to the accompanying drawing in which:

FIG. 1 shows a decoy flare according to an embodiment of the invention,

FIG. 2 shows the spectral and primer flares used in a decoy shown inFIG. 1,

FIG. 3 shows the IR intensity against time for a decoy flare such asshown in FIG. 1,

FIG. 4 shows a primer flare pellet according to an alternativeembodiment of the invention,

FIG. 5 shows a primer flare pellet of a further embodiment of thepresent invention.

Referring to FIG. 1 a primer flare 2 and a spectral flare 4 are housedin an open ended cylindrical flare casing 6. The spectral flare is madeup of a single cylindrical spectral pellet 10 housed at the closed endof the casing whereas the primer flare 2 comprises a stack of primerpellet discs 8 located next to the open end. Each of the primer pelletsdiscs 8 and the spectral pellet 10 are provided with a central hole, 12& 14 respectively, extending throughout the pellets.

The open end of the casing 6 is sealed by a plug 16, the plug beingsecured by crimping the rim 18 of the casing 6 into a groove 24 in theplug 16. An expulsion charge 20 is located in a recess in the outsideface of the plug, as is a primary ignition charge 22.

In use, expulsion charge 20 and primary ignition charge 22 are ignited,for example by conventional electric igniters (not shown) in the flaretube of the target. The expulsion charge 20 is formed from a propellantcompositions such as a gunpowder or nitrocellulose composition and onignition generates a large volume of gas which projects the flare fromthe flare tube (not shown).

Once clear of the flare tube, spring 26 is released and allows theignition stimulus from primary ignition charge 22 to travel down thechannel 28 and ignite the secondary ignition charge 30. Should the flarecasing 6 become jammed in the flare tube, spring 26 is prevented fromrelease and therefore stops the propagation of the ignition stimulus,thereby preventing ignition of the flare in the flare tube.

Ignition of the secondary ignition charge 30 provides a source ofignition for the primer flare 2. The outer and inner surfaces of theprimer flare 2 and spectral flare 4 are also coated in a primer paste tofurther aid ignition. Further, the primer flare 2, and also the spectralflare 4, are wrapped in aluminium foil 32 and 34 respectively, with bothflares being then wrapped together in aluminium foil 38. The aluminiumfoil aids the ignition of the primer flare 2 and spectral flare 4 byinitially confining the ignition gases thus increasing the initialpressure and thereby aiding the speed and reliability of the ignitionstimulus. Wrapping the pellets in aluminium foil also helps to protectthe pellets during storage. The high temperature generated duringburning of the primer flare 2 may additionally cause the aluminium foilto combust, adding to the radiation from the primer flare 2.

Ignition of the primed surfaces of the primer flare 2 cause the primerflare to be very rapidly ignited over all of its outer surface. Hotparticles and combustion gases also travel down the central hole 12igniting the inner surface and the faces between the primer pellet discs8. This is aided by the confining effect of the aluminium foil 32.

The primer pellet discs 8, shown more clearly in FIG. 2; are formed froma fast burning composition such as a composition of MTV. Such acomposition creates an intense source of IR radiation having a fast bumrate and reaches temperatures of 1900° C.

The burn rate of the primer flare is also determined by the thickness ofthe primer pellet discs 8. For an MTV primer pellet composition wherethe discs are of 47 mm diameter and have a central hole of 6 mmdiameter, a stack of 4 to 8 discs of thickness 5-10 mm give the desiredburn rates.

Alternatively the primer pellet could be formed from a similar stack ofdiscs formed from a composition of potassium benzoate and potassiumperchlorate of small particle size and RDX as a binder.

Referring back to FIG. 1, ignition of the primer flare 2 causes ignitionof the spectral flare 4 which is wrapped in aluminium foil 34. Spacers36 are located between the primer flare 2 and the spectral flare 4. Thisallows for hot particles produced from the burning of the primer flare 2to ignite the spectral flare over its surface. The spacers 36 also helpprevent premature ignition due to friction during launch or transit.

In an alternative embodiment (not shown) an ignition transfer medium,such as an MTV cord, could be located next to the secondary ignitioncharge 30 and run through the hole 12 in the primer flare 2 and the hole14 of the spectral flare 4. Ignition of the secondary ignition charge 30could then ignite the ignition transfer medium which would bum down itslength igniting the interior surfaces of the primer flare 2 and thespectral flare 4 in turn. The spectral pellet, also shown in FIG. 2, isformed from a single cylindrical pellet. The composition mayconveniently be a potassium benzoate, potassium perchlorate and Viton Amix. Potassium benzoate comprises 30% by mass of the spectral pelletwith 65% potassium perchlorate and Viton A making up the rest of thepellet. The fuel and oxidant have particle sizes of less then 60microns.

Alternatively the spectral pellet can be formed from a boron, potassiumnitrate, Viton A mix. The potassium nitrate is 70% by mass of thecomposition and has a typical particle size of 100 μm. The boron isamorphous and sub-micron size and makes up 30% by mass of thecomposition or 20% by mass if an additive like silicon is used. If used,the silicon particles are around 10 μm in size. Boron, potassium nitrateand Viton A gives a hard composition which may be easily cast into therequired pellet shapes. Use of a binder such as polybutyl rubber wouldlead to a more flexible composition which could, for example, beextruded.

As the spectral flare 4 is formed from a single cylinder the burn rateis much slower than that of the primer flare 2 and due to the centralhole 14, the intensity of the spectral flare 4 is substantially constantthroughout the duration of burning.

FIG. 3 shows a plot of the IR intensity against time for a decoy flareas shown in FIG. 1 for two fixed IR bands. The primer flare was 47 mm indiameter and 40 mm in length, consisting of 8 discs, each 5 mm thick,formed from an MTV composition. The spectral flare was a single pelletof a potassium benzoate, potassium perchlorate, Viton A mix and was 47mm in diameter and was 110 mm in length. Both the primer flare and thespectral flare were provided with a central hole 6 mm in diameter.

It can be seen that as the primer flare ignites there is a rapidincrease in intensity to a very intense peak which rapidly drops awayagain. The duration of burning of the primer flare can be seen to be inthe region of 200 ms. It can also be seen that one IR band is very muchmore intense than the other. This is the situation with conventionalflares but not with targets such as aircraft. After the primer flare hasfinished burning it can be seen that the spectral flare is alreadyburning and that the intensity of both bands drop off to a lower level.However, the band which was, during burning of the primer flare, oflower intensity is now of greater intensity, which is the oppositesituation to what would be expected for a conventional decoy and issimilar to what the output from the target would be. This occurs a fewhundred milliseconds after the primer flare ignited and therefore solelythe spectral flare will be burning by the time the spectraldiscriminator of a missile's CCM system will have been activated.

It can be seen that the spectral flare burns with relatively consistentintensity in both bands for a few seconds, more than long enough for thetarget to be well outside the missile's field of view when the flarefinally stops burning.

FIGS. 4 & 5 show alternative embodiments of a primer flare suitable foruse in decoy flare according to the present invention. Where appropriatelike numerals have been used to designate like components.

Referring to FIG. 4 the primer flare 2 is formed from a singlecylindrical pellet 42 having a central hole 12. The pellet 42 is alsoprovide with a number of other holes 44 which pass through the pellet,the inside surfaces of the central hole 12 and the holes 42 being coatedwith a primer paste to aid in ignition. The use of a single pellet witha number of holes can simplify the production of the primer flare as asingle pellet can be easily machined, however the loss of material fromthe primer flare could necessitate the use of a larger pellet, dependingupon the application.

For a 47 mm diameter and 40 mm long primer flare made from an MTVcomposition the desired burn rate can be achieved with one central hole12 and five surrounding holes 42, each hole being of 6 mm diameter.Additionally or alternatively transverse holes could be provided downthe length of the primer pellet.

FIG. 5 shows an a rectangular primer flare 50 comprising a single pelletwith a central hole 52 and a number of grooves 54 running down thelength of the pellet. The grooves may be a few mm thick and about 15 mmdeep depending upon the application and may be filled with primer pasteto aid ignition.

1-26. (canceled)
 27. An infra-red emitting decoy flare comprising aflare casing, two pyrotechnic components housed within the flare casing,and an ignition system for igniting the pyrotechnic components, the twopyrotechnic components comprising a primer flare and a spectral flare,the primer flare including at least one primer pellet comprising a fastburning pyrotechnic composition that causes rapid ignition of the primerpellet to produce an intense infra-red source, the spectral flareincluding at least one spectral pellet comprising a pyrotechniccomposition that produces a spectral infra-red source having ratios ofthe intensity of the infra red spectrum measured at at least two fixedbands at the same time lying within a fixed range, and wherein thespectral flare is still burning after the primer flare has finishedburning, and wherein the spectral flare further comprises an explosivematerial with a waxy composition.
 28. An infra-red emitting decoy flareas claimed in claim 27 wherein the intensity of the infra-red radiationemitted from the spectral flare is greater than that normally emittedfrom an intended target.
 29. An infra-red emitting decoy flare asclaimed in claim 27 wherein the primer flare comprises a stack of discsof a pyrotechnic composition.
 30. An infra-red emitting decoy flare asclaimed in claim 27 wherein the primer flare is provided with a centralhole.
 31. An infra-red emitting decoy flare as claimed in claim 27wherein the primer flare is provided with a plurality of holes.
 32. Aninfra-red emitting decoy flare as claimed in claim 27 wherein the primerflare has a plurality of grooves in its surface.
 33. An infra-redemitting decoy flare as claimed in claim 27 wherein the burn time of theprimer flare is within the range of 100 to 600 ms.
 34. An infra-redemitting decoy flare as claimed in claim 33 wherein the burn time of theprimer flare is in the range of 150 to 250 ms.
 35. An infra-red emittingdecoy flare as claimed in claim 27 wherein the spectral flare comprisesa single pellet.
 36. An infra-red emitting decoy flare as claimed inclaim 35 wherein the spectral flare is provided with a central hole. 37.An infra-red emitting decoy flare as claimed in claim 27 wherein thespectral flare is ignited by the burning of the primer flare.
 38. Aninfra-red emitting decoy flare as claimed in claim 27 wherein the primerflare is formed from a fast burning magnesium-Teflon-Viton composition.39. An infra-red emitting decoy flare as claimed in claim 27 wherein theignition of the primer pellet produces an intense spectral infra-redsource wherein the ratio of the intensity of the infra-red spectrum atat least two fixed bands is within a fixed range.
 40. An infra-redemitting decoy flare as claimed in claim 39 wherein the primer flare iscomprised from a composition of potassium benzoate, potassiumperchlorate and a binder comprised of an explosive material with a waxycomposition.
 41. An infra-red emitting decoy flare as claimed in claim27 wherein the spectral flare pyrotechnic composition producesquantities of gas on ignition.
 42. An infra-red emitting decoy flare asclaimed in claim 27 wherein the spectral flare is formed from acomposition having an organic fuel or a boron fuel, an oxidant and abinder.
 43. An infra-red emitting decoy flare as claimed in claim 42wherein the oxidant is chosen from the group of potassium perchlorate,potassium nitrate, sodium nitrate and ammonium nitrate.
 44. An infra-redemitting decoy flare as claimed in claim 42 wherein the binder is chosenfrom the group of Viton A, dextrin or polybutyl rubber.
 45. An infra-redemitting decoy flare as claimed in claim 42 wherein the spectral flarecomposition comprises an additive such as silicon.
 46. An infra-redemitting decoy flare as claimed in claim 42 wherein the organic fuel ischosen from the group comprising sucrose, lactose, starch and potassiumbenzoate.
 47. An infra-red emitting decoy flare as claimed in claim 27wherein the explosive material is chosen from the group of RDX, HMX andHNS.
 48. An infra-red emitting decoy flare as claimed in claim 27wherein the spectral flare composition is formed from a mix of 30-40% bymass of potassium benzoate and 60-70% by mass of potassium perchloratetogether with 3-8% by mass of RDX, HMX or HNS.